[Webkit-unassigned] [Bug 139106] New: Generalize PageActivityAssertionToken

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=139106

            Bug ID: 139106
           Summary: Generalize PageActivityAssertionToken
    Classification: Unclassified
           Product: WebKit
           Version: 528+ (Nightly build)
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebCore Misc.
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: barraclough at apple.com

PageActivityAssertionToken is a RAII mechanism implementing a counter, used by PageThrottler to count user visible activity in progress on the page (currently page load and media playback). Use of an RAII type is prevents a number of possible errors, including double counting a single media element, or failing to decrement the count after a media element has been deallocated.

Generalizing the mechanism would both make it available for new uses (currently I want to track page load separately from media playback in PageThrottler, and track processes with active pages in WK2), and also may provide a better implementation for some existing accounting mechanisms (tracking processes suppression state across processes/contexts/globally in WK2, tracking outstanding user visible resource requests).

The current implementation has a number of drawbacks that have been addressed by this refactoring:
 - specific to single use in PageThrottler class - not reusable.
 - incomplete encapsulation - the counter and WeakPtrFactory that comprise the current implementation are not encapsulated (are in the client type, PageThrottler).
 - tokens are not shared - PageActivityAssertionToken instances are managed by std::unique, every increment requires an object allocation.
 - redundancy - the current implementation uses a WeakPtr to safely reference the PageThrottler, this is internally implemented using a reference counted type, resulting in two counters being incremented (one in the PageActivityAssertionToken, one in the PageThrottler).

In the reimplementation:
 - a callback is provided via a lambda function, which allows for easy reuse without a lot of boilerplate code.
 - the counter, callback and ownership of the otherwise weakly-owned token is encapsulated within the RefCounter type.
 - a single count within RefCounter::Counter stores the counter value, and also manage the lifetime of this object.
 - standard RefPtrs are used to manage references to the RefCounter::Counter.

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